IEC Lead Continuity looks for the IEC lead connection whereas the Earth Continuity looks for the earth bond lead. Therefore, if you check an IEC lead with the earth continuity option it will be open circuit everytime.

The Seaward 110v adaptor does have the 110v Line (miss called live) connected to the neutral of the 13A plug. This prevents 110v equipment being connected to 230v mains whilst still allowing insulation testing to be carried out. However, as you have discovered, it prevents lead continuity/polarity tests.

I use an appropriately lablled straight connected adaptor so I can run lead tests.

110v extension cables will normally be 3 core cable with wiring to all three pins L N and E at both ends. The tests involved would be visual, earth bond test at 25A, insulation test at 500v and polarity check. I dont know about the PT300 but on the supernova you plug one end of the lead directly into the tester and the other end into the IEC port via an adapter lead. Your earth bond pass limit will obviously vary dependent upon cable CSA and lead length.

To clarify I made up my own 110V cee-form to IEC adaptor which is marked up only for use as a specialist test adaptor.